Growing concerns about climate change induced by anthropogenic CO2 accumulation has lead to interests in technologies that slow the growth of CO2 output or in sonic of the most ideal scenarios, halt the increase in atmospheric CO2 concentration. Ideally, society would rapidly switch to renewable energy sources such as wind, solar geothermal and biomass-derived energy because these non-fossil fuels based energy sources are carbon-neutral. However, given the scale of society's energy needs, it will be many decades before renewable fuels can bear the majority of the burden of powering our planet. In the meantime, fossil energy will continue to supply the bulk of the energy demand. In such a scenario, the atmospheric CO2 level will continue to climb above the current concentration of ˜390 ppm.
One approach to capture CO2 from fossil fuel combustion is to trap CO2 at large point sources such as electricity generating power plants. However, roughly ⅓ of global carbon emissions are associated with distributed sources such as transportation fuels. Thus, large-scale deployment of carbon capture and sequestration (CCS) technologies at various point sources can at best slow the rate of increase of the atmospheric CO2 concentration. What is needed is a carbon-negative technology, one that actually reduces the concentration of CO2 in the atmosphere. Here, building on the limited existing work, we propose the direct capture of carbon dioxide from the ambient air using solid adsorbents specifically designed for this task as a new approach for reducing the concentration of CO2 in the ambient air. The captured CO2 could be sequestered semi-permanently, for example, underground, or used in a beneficial application, such as a feedstock for algae-based biofuels production.
When considering the possible ways of removing CO2 from a gas mixture, several approaches can be considered, including cryogenic distillation, membrane purification, absorption with liquids and adsorption using solids. These methods are typically designed for concentrated CO2 sources in the large-scale removal processes, such as flue gas treatment from large point sources including fossil fuel burning power plants or from natural gas streams contaminated with CO2. For separation of CO2 from very dilute sources, such as the ambient air, (e.g., approximately 390 ppm) many traditional methods such as cryogenic distillation and application of membranes are not expected to be cost-competitive. Keith and Dodge have demonstrated that classical absorption processes using basic chemicals such as alkali metal hydroxides dissolved in water can be used to extract CO2 from the ambient air. However, this approach still appears to be quite expensive. Adsorption processes based on application of solid sorbents may be an alternate approach that is more cost-effective.
Many different types of solid adsorbents have been applied to CO2 capture from flue gas and other moderately concentrated sources, (5-20% CO2). However, when considering the unique constraints of direct “air capture” of CO2 [the ubiquitous presence of water, low temperature operating window (ambient temperature to ˜120° C.), dilute feed source (370-425 ppm)], many classes of adsorbents can be immediately ruled out. For example, all the physisorbents such as activated carbons and zeolites will suffer from overwhelming competitive adsorption of water over CO2 in most cases. In addition, because the heat of adsorption on these solids is so low, physisorbents will have very low CO2 adsorption capacities at the CO2 concentration found in ambient air. In contrast, many of the chemisorbents such as calcium oxides and lithium zirconates require high temperature operating conditions above 300° C. Thus, these materials will not be useful in low-temperature air capture processes. On the other hand, chemisorbents based on solid-supported organic amines have properties such as low operating temperature and high heat of adsorption that make them potentially ideal sorbents for direct capture of CO2 from the ambient air. We had found that hyperbranched aminosilica (HAS) adsorbents are highly useful as new effective adsorbents for CO2 separation from mildly concentrated sources such as flue gas. In a follow-up study, we also showed that the adsorption properties such as CO2 capacities and adsorption kinetics could be tuned by the synthesis conditions. There arc three classes of supported amine adsorbents (defined as those that employ any of primary, secondary, tertiary or mixture of these types of amine sites) reported to date:                (i) those based on impregnation (physical adsorption) of amine-containing small molecules or preformed oligomeric/polymeric amines into porous supports;        (ii) those based on covalent binding of amines to the surface of a support through use of silane coupling agents; and        (iii) those based on in-situ surface polymerization of reactive amine monomers, creating covalently-bound amines to the silica substrates.        
Previously Birbara et al. developed a class (i) supported amine adsorbent on a polymeric substrate such as acrylic esters, denoted as TEPAN from the reaction of tetraethylenepentamine (TEPA) and acrylonitrile (AN), targeting CO2 capture from dilute gases to maintain a breathable air stream, for example on a space station. However, although this class (i) material enhances the regenerable cyclic CO2 absorption capacity by more than two and half times compared to use of TEPA, this report lacks a detailed description showing the sorbents CO2 capture performance, and no data are provided for ambient air conditions. In a series of patents, Wright et al. proposed using strong base ion exchange resins (quaternary ammonium compounds) as a means for direct CO2 capture from the air. Although these claims primarily focused on more efficient designs of air capture devices involving anion exchange solids, it seems that the actual CO2 capture performance needs further improvement. For example, average capture fluxes of 0.02˜0.06 mmol CO2/m2/sec suggests this air collecting apparatus must have very large dimensions. Riecke et al. employed amine-containing hollow fiber membranes for CO2 separation from breathing gas mixtures, while reporting a selectivity of at least 500 for CO2 relative N2. In this report, the CO2 transportation mechanism was explained by the reversible formation of HCO3 from CO2 and H2O, suggesting that humidity is indispensible for its operation. As a result, the authors described that this membrane exhibited extremely low transport capacity for CO2 under dry ambient conditions. Furthermore, this is a membrane technology, not an adsorbent technology.
The aforementioned disclosures were rather broadly drawn but short on significant details on atmospheric CO2 capture. Olah et al. provided information on the absorption performance of several solid sorbents from an experiment using gas mixtures of 370 ppm CO2, 80% N2, and 20% O2. In this experiment, the powdered nanostructured silica absorbent comprising 47.5% polyethylenimine (PEI) and 10% polyethylene glycol (PEG) displayed a CO2 absorption capacity of ˜0.6 mmol/g at a temperature of 323 K. It should be noted that the supported amine sorbents described in these reports were prepared by simply mixing an amine solution with nonporous silica nanoparticles as the support (e.g., fumed silica with particles smaller than ˜100 nm), creating no covalent linkage of the organic amines to the support.
Apart from the above-mentioned approaches, there have been attempts to employ the amidourea macrocycles as a CO2 catching media. For example, Brooks et al. presented an experimental result suggesting that this macrocycle, when dissolved in dimethyl sulfoxide and tetrabutyl ammonium fluoride, enables atmospheric CO2 fixation. Recently, Tossell explained by the computational method that CO2 forms a complex in which a CO3 agroup is held by a number of O—H—N bonds within the bowl-shaped cavity of the macrocyclic amidourea. Although it was suggested that such macrocycles might capture CO2 from the ambient air, no experimental or simulation results verify this assertion. Furthermore, the macrocycle is hard to synthesize, not environmentally benign, and likely less efficient than conventional sorbents in terms of the capture capacity.